One or More Species of Pacific Tree Frogs? Insights From Vocal Sexual Signals

Evolutionary divergence in sexual signals may lead to or maintain reproductive isolation between populations. Both selective forces-such as ecological and sexual selection-and random processes like genetic drift may influence the diversification of sexual signals. Understanding the patterns and sources of intraspecific variation in sexual signals among populations can inform the stages of differentiation and speciation. In this study, we investigated patterns of geographic variation in vocal sexual signals and how they relate to genetic and environmental distances among nine populations of Pacific tree frogs. Importantly, the taxonomy of Pacific tree frogs remains unresolved; while some authors recognize only one species (Pseudacris regilla), other authors propose three distinct species based on mitochondrial DNA lineages (P. regilla, P. sierra, and P. hypochondriaca). Our genetic analyses revealed that the nine populations studied span two of the three mitochondrial lineages of Pacific tree frogs. We found that variation in the advertisement calls is better explained by mitochondrial lineage than by geographic or environmental distances between populations. The acoustic properties that have diverged the most between lineages relate to the number of pulses in the call and the rate at which the pulses are delivered. Interestingly, these acoustic properties are important for species recognition in this and other species of frogs. These findings suggest that differences in the vocal sexual signal may lead to premating reproductive isolation between mitochondrial lineages of Pacific tree frogs.